1. Field of the Invention
The invention relates to a crucible assembly and a method of manufacturing a crystalline silicon ingot casting by use of such crucible assembly, and particularly to a crucible assembly capable of lowering impurities and oxygen content and reducing red zone of a crystalline silicon ingot fabricated by such crucible assembly. Moreover, the photoelectric conversion efficiency of photovoltaic cell made from the crystalline silicon ingot fabricated by the crucible assembly is improved significantly.
2. Description of the Prior Art
Most of the photovoltaic cells produce a photovoltaic effect when absorbing sunlight. Currently, the photovoltaic cell is made of a silicon-based material, since for the most parts; silicon is the second most abundant and accessible element on Earth. Also, silicon is cost-effective, nontoxic, and chemically stable, and becomes broadly used in semiconductor applications.
There are three forms of crystalline silicon for fabricating silicon-based photovoltaic cells, i.e., mono-crystal silicon (mono-Si), multi-crystal or poly-crystal silicon (poly-Si), and amorphous silicon (a-Si). Multi-crystal or poly-crystal silicon is much less expensive than mono-crystalline silicon when produced by Czochralski (CZ) method or float zone (FZ) method, so it is usually used as a raw material of the photovoltaic cell due to the economic concern.
Conventionally, multi-crystal or poly-crystal silicon for photovoltaic cells is fabricated by a common casting process. That is, it is prior art to produce multi-crystal or poly-crystal silicon for photovoltaic cells by a casting process. In brief, the multi-crystal or poly-crystal silicon photovoltaic cell is fabricated by melting high purity silicon in a mold such as a quartz crucible, then cooling the melted silicon in a controlled solidification to form a multi-crystal or poly-crystal silicon ingot. The multi-crystal or poly-crystal silicon ingot is generally cut into bricks having a cross-section that is the same as or close to the size of the wafer to be used for manufacturing a photovoltaic cell, and the bricks are sawed or otherwise cut into such wafers. The ploy-Si produced in such manner is an agglomeration of crystal grains where, within the wafers made therefrom, the orientation of the silicon grains relative to one another is practically random.
The random orientation of silicon grains, in either conventional multi-crystal or poly-crystal silicon, makes it difficult to texture the surface of a resulting wafer. Texturing is used to improve efficiency of a photovoltaic cell, by reducing light reflection and improving light energy absorption through the surface a cell. Additionally, “kinks” that form in the boundaries between the grains of conventional multi-crystal or poly-crystal silicon tend to nucleate structural defects in the form of clusters or lines of dislocations. These dislocations, and the impurities they tend to attract, are believed to cause a fast recombination of electrical charge carriers in a functioning photovoltaic cell made from conventional multi-crystal or poly-crystal silicon. This can cause a decrease in the efficiency of the cell. Photovoltaic cells made from such multi-crystal or poly-crystal silicon generally have lower efficiency compared to equivalent photovoltaic cells made from mono-crystal silicon, even considering the radial distribution of defects present in monocrystalline silicon produced by known techniques. However, because of the relative simplicity and lower costs for manufacturing conventional multi-crystal or poly-crystal silicon, as well as effective defect passivation in cell processing, multi-crystal or poly-crystal silicon is a more widely used form of silicon for manufacturing photovoltaic cells.
Currently, it has been developed that crystalline silicon ingot is fabricated using a mono-crystal silicon seed layer and based on a directional solidification. In this way, a high quality ingot of mono-crystalline silicon and/or bi-crystal silicon block or mono-like crystal silicon block may be obtained, in which the lifetime of the minority carriers is maximized in the resultant wafer employed for fabricating a high-performance photovoltaic cell. As used herein, the term “mono-crystal silicon” refers to a body of single crystal silicon, having one consistent crystal orientation throughout. The term “bi-crystal silicon” refers to a body of silicon, having one consistent crystal orientation throughout for greater than or equal to 50% by volume of the body, and another consistent crystal orientation for the remainder of the volume of the body. For example, such bi-crystal silicon may include a body of single crystal silicon having one crystal orientation next to another body of single crystal silicon having a different crystal orientation making up the balance of the volume of crystalline silicon. The term “mono-like crystal silicon” refers to a body of silicon, having one consistent crystal orientation throughout for greater than 75% by volume of the body. Additionally, conventional multi-crystal silicon refers to crystalline silicon having cm-scale grain size distribution, with multiple randomly oriented crystals located within a body of silicon. The term “poly-crystal silicon” refers to crystalline silicon with micron order grain size and multiple grain orientations located within a given body of silicon. For example, the grains are typically an average of about submicron to sub-millimeter in size (e.g., individual grains may not be visible to the naked eye), and grain orientation distributed randomly throughout.
It has also been developed that crystalline silicon ingot is fabricated using a nucleation promotion layer constituted by granulars of mono-crystal or poly-crystal silicon to assist in nucleation of silicon grains and based on a directional solidification. The resultant crystalline silicon ingot has small-sized silicon grains at the bottom thereof and low density of bulk defects, and can be used for fabricating a high-performance photovoltaic cell. By the nucleation promotion layer of small-sized crystal particles, the fineness of silicon grains can inhibit the growth of dislocations to reduce the opportunity of dislocation growth. The photoelectric conversion efficiency of photovoltaic cell made from such crystalline silicon ingot is quite high.
The region in crystalline silicon ingot, which is generally fabricated using crucible and does not meet requirement, represents as “red zone”. The photovoltaic cell made from the red zone of general crystalline silicon ingot has low minority carrier lifetime. The causes of red zone include: a. the region containing impurities in solid state diffusion from crucible; b. the region of non-perfect crystal structure near the crucible; c. the boron-rich or oxygen-rich region; and d. the crystalline seeds and nucleation layer containing metal in liquid state diffusion from the silicon melt containing metal. In general, the region containing impurities, especially metal impurities, in solid state diffusion from crucible is the primary cause of red zone. The photoelectric conversion efficiency of photovoltaic cell made from the red zone of crystalline silicon ingot severely decays and decreases.
The red zone of current crystalline silicon ingots, fabricated using a layer of single crystal seeds or a nucleation promotion layer of mono-crystal or poly-crystal silicon granulars disposed at the bottom of crucible, is larger than or even twice as much as that of crystalline silicon ingots fabricated using no above layers. Studying its causes, during fabrication of the crystalline silicon ingot, impurities primarily consisting of metal impurities (e.g., Fe, Al, etc.) in the crucible are dissolved in the silicon melt, and then diffuse into single crystal seeds or mono-crystal or poly-crystal silicon granulars. As silicon grains from the silicon melt nucleate and grow at the single crystal seeds or the mono-crystal or poly-crystal silicon granulars, the impurities in the polluted single crystal seeds or the polluted mono-crystal or poly-crystal silicon granulars will diffuse back into solidified silicon crystals.
Chinese publication no. CN102776554A, filed by LDK SOLAR Corp., discloses a method of fabricating a polycrystalline silicon ingot. The method of CN102776554A includes the step of coating a layer of silicon nitride on the inner wall of a crucible where another function the layer of silicon nitride is used as the releasing agent preventing the polycrystalline silicon ingot from sticking the crucible during cooling and resulting in releasing failure. A layer of porous material is disposed on the bottom of the crucible, and then, a silicon feedstock is filled on the layer of porous material. Afterward, the method of CN102776554A is to heat the crucible to melt the silicon feedstock into a silicon melt, and then, to solidify the silicon melt into the polycrystalline silicon ingot on the basis of a directional solidification. During fabrication of the polycrystalline silicon ingot, the layer of silicon nitride coated on the inner wall of crucible can prevent impurities in the crucible from diffusing into the silicon melt and the polycrystalline silicon ingot to enhance the quality of the polycrystalline silicon ingot. The layer of porous material is a porous plate which is formed by sintering of silicon nitride, silicon carbide or quartz. The pores of the porous plate are extrinsic, formed by molding and regularly arranged. The pores of the porous plate can assist silicon grains in nucleation during fabrication of the polycrystalline silicon ingot. In the published specification of CN102776554A, it is issued that the method achieves well initial nucleation of silicon grains to effectively control the growth of dendrites, to lower multiplication of dislocations during growth of the polycrystalline silicon ingot, and to obtain the polycrystalline silicon ingot with good quality.
However, the layer of porous material disclosed in CN102776554A is a layer of rigid and porous material. The crystalline silicon ingot after nucleation and growth has plasticity at a high temperature (about 800° C.), and so has stress caused by the silicon melt pressing on the top of the crystalline silicon ingot. Because the layer of rigid and porous material cannot relieve the stress in the crystalline silicon ingot, there is still much room for improvement in inhibiting increasing of defects such as dislocations for manufacture of a crystalline silicon ingot by use of a layer of porous material.
In addition, the layer of porous material disclosed in CN102776554A provides regularly arranged pores which nucleate silicon grains unlike the silicon grains nucleated by randomly arranged pores. The silicon grains nucleated by the randomly arranged pores can use stress field to attract defects to agglomerate or use slip on the grain boundaries to release thermal stress to inhibit rapid increasing of defects such as dislocations. In addition, in the specification of CN102776554A, the layer of silicon nitride on the inner wall of the crucible can prevent impurities in the crucible from diffusing into the silicon melt and the polycrystalline silicon ingot, but it still has much room for improvement in lowering defect density in the polycrystalline silicon ingot. Moreover, the method of CN102776554A cannot effectively assist the crystalline silicon ingot in reducing oxygen content.
In view of the foregoing problems of prior arts, there is no crucible capable of lowering cost, assisting in releasing therefrom, and effectively assisting in manufacturing a crystalline silicon ingot which reduces impurities, defects such as dislocations and oxygen content therein and reduces red zone thereof. The photoelectric conversion efficiency of photovoltaic cell made from such crystalline silicon ingot will enhance significantly.